The invention relates to an image display device comprising at least one two-dimensional display element and an optical system, which optical system comprises a lens system having a first semitransparent reflecting surface which is directed towards the display element and a second semitransparent reflecting surface which is remote from the display element.
The invention also relates to a head-mounted display comprising such an image display device.
An image display device of the type described in the opening paragraph is known from, for example United States patent U.S. Pat. No. 5,659,430. The image display device described in this patent comprises a two-dimensional display element and an optical system via which the image generated by the display element is imaged in the eye of a viewer. The optical system comprises a buried mirror lens. Such a lens has a first and a second semitransparent reflecting surface on which incident light rays are passed at least once and reflected at least once. In this way, it is achieved that the light path between the display element and the viewer's eye can be considerably shortened by repeated reflections.
An image display device as described above is very suitable for use in head-mounted displays. Compactness and weight are of great importance in head-mounted displays. Important factors are the weight and the volume of the optical system. Moreover, it is desirable that a large field of view can be realized in head-mounted displays, notably for virtual reality applications.
A drawback of the image display device described is that the display element must be relatively large if an optimal maximum resolution is to be achieved, so that the cost price is relatively high. Another drawback is that, when the display element is a reflective display panel, coupling in light is impeded because the on-going and projected beams propagate along the same light path. A solution to this problem is the use of a splitting cube or a splitting mirror, but this is at the expense of the compactness of the system in relatively large display elements.